A dental nurse tried to poison her boss out of “spite or revenge” after receiving warnings about her poor performance, a court has heard.

Ravinder Kaur, 34, put mercury amalgam – a substance used in dental fillings – into a cup of coffee she made for Laura Knowles at the dental practice in Shefford, Bedfordshire, on 16 March last year, Blackfriars Crown Court heard. Ms Knowles thought the drink tasted strange before finding “grey sludge” at the bottom. She was sick and suffered stomach cramps.